Short-stroke position transducer for a vehicle suspension system

ABSTRACT

A vehicle suspension system (10) has a spring (11) operatively arranged to act between sprung (15) and unsprung (13) masses. The coil (24) of an LVDT (22) is mounted on the sprung mass. The core (23) of the LVDT is connected to an intermediate portion (25) of the spring. Thus, displacement of the core relative to the coil will be a proportional fraction of the relative displacement between the masses. This arrangement allows the use of a short-stroke position-sensing transducer to measure the relative displacement between the two masses.

TECHNICAL FIELD

The present invention relates generally to the field of vehiclesuspension systems, and, more particularly, to an improved arrangementwhich allows the use of a short-stroke transducer to sense the relativepositions between the sprung and unsprung masses of a vehicle.

BACKGROUND ART

In a conventional suspension system, a road wheel is mounted formovement upwardly and downwardly relative to the body. A coil spring anda shock absorber are typically arranged between the body and thesupporting structure on which the wheel is mounted. The shock absorbercontrols the flow of fluid between two chambers, and functions to dampenthe velocity of relative movement between the body and the road wheel.

In recent years, there has been considerable interest in developing an"active" vehicle suspension system. See, e.g.: "Lotus' activesuspension", Automotive Engineer (Febrary/March 1984) [pp. 56-57];McCosh, "no-springs, no-shocks suspension", Popular Science (July 1986)[pp. 60-63]; and McCosh, "Springless Corvettes", Popular Science (Sept.1986) [p.12]. In these "active" systems, the conventional spring andshock absorber are replaced by a servocontrolled double-acting hydraulicactuator. Suitable sensors, such as accelerometers and the like, areused to sense and determine certain parameters, such as body attitudeand acceleration. The signals generated by these sensors areappropriately tailored and used as command signals to servovalves, whichcontrol the flow of fluid to the actuators. Thus, rather than merelyresponding passively and reactively to various forces acting on thevehicle, the "active" systems may be used to affirmatively correct andcompensate for the effects of such forces. For example, during braking,the conventional vehicle will "nose down". However, with anactively-controlled system, the various servos may be operated, eitherindependently or in combination with one another, to maintain the bodylevel and horizontal during braking, acceleration and cornering.

In some hybrid or semi-"active" systems, the conventional springremains, but the shock absorber is replaced by a servocontrolledactuator. Such spring and actuator may be arranged either in parallel orin series with one another.

In any event, in such "active" and "semi-active" systems, and in stillother level- and attitude-control systems (e.g., those employingvariable-rate shock absorbers having fluid damping characteristics whichvary with position), it is often desired to known the relative positionsbetween the wheel and body. Such data may, for example, be used as afeedback signal in a closed-loop servocontrol system. There are manydevices available for sensing position and converting it into an analogelectrical signal. One such device is a linear variable differentialtransformer (LVDT), which has a core movable relative to a sensing coil.Such devices could, of course, be connected directrly to the body andthe wheel. However, the typical stroke of relative movement between thewheel and the body is relatively long, and this would require the use ofa correspondingly long-stroke LVDT.

DISCLOSURE OF THE INVENTION

The present invention provides a unique improvement for use in a vehiclesuspension system. The vehicle may be, but is not limited to, anautomobile, truck, track-laying vehicle, or the like. A spring isoperatively arranged between the sprung and unsprung masses. The springmay have a constant spring rate along its entire length, or differentsections thereof may have different rates, as when two different springsare mechanically connected in series with one another. In any event, thespring may either be a working component of the suspension or levelingsystem, or may be provided solely for the purposes of the sensingposition. The improvement broadly provides a transducer, such as anLVDT, for sensing the position of two relatively-movable transducerparts, and for converting such sensed position into an equivalentelectrical signal. One part of this transducer is mounted for movementwith either the sprung or unsprung masses, and the other part is mountedfor movement with an intermediate portion of the spring. The ratio ofthe distance between the mass to which the transducer one part isconnected and the point of the spring to which the transducer other partis connected, to the length of the spring, determines a degree of motionreduction. Thus, relative movement between the ends of the spring willproduce a smaller but proportional displacement of the tworelatively-movable transducer parts. The improvement allows the use ofrelatively short-stroke sensing devices to measure the relativepositions of the two masses.

Accordingly, the general object of the invention is to provide animproved device for sensing the position of two relatively-movablemasses in a vehicle suspension system.

Another object is to provide an improvement for use in a vehiclesuspension system, which allows the use of short-stroke sensingtransducers to sense and determined the relative positions between twomasses.

Another object is to provide an improvement for use with a system forcontrolling the level or attitude of a vehicle body relative to a roadwheel.

These and other objects and advantages will become apparent from theforegoing and ongoing specification, the drawings, and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an improved vehicle suspension system,showing a spring and shock absorber arranged to act between the body anda wheel of a vehicle, and also showing a transducer as being operativelyarranged between the body and an intermediate portion of the spring.

FIG. 2 is an enlarged detail view of the transducer and spring shown inFIG. 1.

FIG. 3 is a view similar to FIG. 2, but illustrating theproportionately-smaller displacement of the relatively-movabletransducer parts in response to an upward displacement of the road wheelrelative to the body.

MODES(S) FOR CARRYING OUT THE INVENTION

At the outset, it should be clearly understood that like referencenumerals are intended to identify the same structural elements, portionsor surfaces consistently throughout the several drawing figures, as suchelements, portions or surfaces may be further described or explained bythe entire written specification, of which this detailed description isan integral part. Unless otherwise indicated, the drawings are intendedto be read (e.g., cross-hatching, arrangement of parts, etc.) togetherwith the specification, and are to be considered a portion of the entire"written description" of this invention, as required by 35 U.S.C. §112.As used in the following description, the terms "horizontal","vertical", "left", "right", "up" and "down", as well as adjectival andadverbial derivatives thereof (e.g., "horizontally", "rightwardly","upwardly", etc.), simply refer to the orientation of the illustratedstructure as the particular drawing figure faces the reader.

This invention provides an improvement for use in a vehicle suspensionsystem, of which a fragmentary schematic portion is generally indicatedat 10 in FIG. 1. This suspension system is shown as including, inpertinent part, a coil spring 11 and a servocontrolled double-actinghydraulic actuator 12 operatively interposed between a support 13 onwhich wheel 14 is mounted and the body 15 of a vehicle. The spring andactuator are shown as being arranged in parallel, but this configurationneed not invariably obtain. Persons skilled in this art will readilyappreciate that the wheel-supporting device 13 is suitably mounted, viacontrol arms and the like (not shown), for upward and downward movementrelative to the body. The specific structure by which such device ismounted on the body, has been omitted in the interest of clarity. Thus,spring 11 is shown as having its upper end portion 16 arranged to actagainst a portion of the body, and as having its lower end portion 18arranged to act against the wheel-supporting device 13.

Actuator 12 is shown as being a conventional double-acting hydraulicactuator arranged to act between the body and the support. The specificdetails of the servocontrol (e.g., the servovalve, the fluid source, andthe like) have been omitted in the interest of clarity. Actuator 12 issimply shown as having a cylinder 19 mounted on the wheel-supportingdevice, and having a piston 20 slidably mounted within this cylinder.The piston is connected to the body via a rod 21. Thus, fluid may beselectively permitted to flow to and from the opposed actuator chambersto vary the relative positions of the wheel and body.

The present invention provides a unique improvement for use in such avehicle suspension system, and allows the use of a short-stroketransducer to sense the relative position of the two relative-movablelong-stroke masses.

Referring now to FIGS. 2 and 3, the improvement includes a transducer22, such as an LVDT or the like, for converting position into an analogelectrical signal. The transducer has a core 23 mounted for movementrelative to a sensing coil 24. Coil 24 is mounted to the body 15 of thevehicle for movement therewith. However, the core 23 is not connecteddirectly to the road wheel assembly. Rather, the core is connected to anintermediate portion, indicated at 25, of spring 11. Thus, if the springhas a vertical length "L", the portion of the spring between the bodyand the point (i.e., 25) at which the core is connected, is representedby the vertical distance "a", and the balance of the spring between thispoint and the wheel support is represented by vertical distance "b".Thus,

    L=a+b

The significance of this arrangement is that the vertical displacementof the core relative to the coil will be a fraction of the displacementof the road wheel relative to the body. In other words, if the roadwheel assembly 13 moves upwardly by a distance "X", as by hitting abump, the transducer core will move upwardly relative to the coil, by aproportionally-smaller distance "s_(t) " according to the formula:

    x.sub.t =aX/L

This, of course, assumes the absence of friction, and that suchcompressive displacement of the spring occurs uniformly over its entirelength. The foregoing equation may be readily solved for "a":

    a=x.sub.t L/X

Thus, for example, if the spring has a nominal length of, say, 10 unitswhen X =0, if the displacement of the road wheel relative to the bodyis, say, x=±6 units, and if it is desired to use a short-stroke LVDThaving a measuring range of, say, x_(t) =±1.75 units, then:

    a=(1.75)(10)/(6)=2.92 units

Thus, the invention provides a unique improvement for use with a vehiclesuspension system, and particularly with an "active" system, whichallows the use of a short-stroke transducer to measure the displacementof one mass, such as a control arm or the like, relative to anothermass, such as the body of a vehicle.

Modifications

The present invention contemplates that many changes and modificationsmay be made. As previously noted, the spring itself may be a workingcomponent of the suspension system, or a passive "measuring-only"element. The spring may be a coil spring, a leaf spring, a torsionalspring or bar, a flexure member, or some other type. Such spring may beconnected between the body and the device on which the wheel is mounted,or between other relatively-movable parts. The spring may have a uniformspring rate along its entire length, or may have a variable spring rate,as, for example, if two different springs were mechanically connected inseries. The transducer may be an LVDT, or some other type ofposition-sensing device. While one part of such device must be connectedto an intermediate portion of the spring, the other part thereof may beconnected to either the sprung or the unsprung mass. As used in theappended claims, the term "vehicle" includes, but is not limited to, anautomobile, a truck, a track-laying vehicle, and the like. It will bereadily apparent that the improvement may be provided between the bodyand each separate wheel-supporting device, as desired. Moreover, theimprovement may be incorporated in other body-leveling orattitude-control systems.

Therefore, while the presently-preferred embodiment of the improvementhas been shown and described, and several modifications thereofdiscussed, persons skilled in this art will readily appreciate thatvarious additional changes and modifications may be made withoutdeparting from the spirit of the invention, as defined anddifferentiated by the following claims.

I claim:
 1. In a vehicle suspension system having a first mass and a second mass, and wherein a spring is operatively arranged between said masses, the improvement which comprises:a transducer operatively arranged to continuously sense the relative position of said masses and to continuously produce an electrical signal substantially proportional to such sensed relative position, said transducer having two relatively-movable parts, one of said parts being mounted for movement with one of said masses and the other of said parts being mounted for movement with an intermediate portion of said spring; whereby, in the event of relative movement between said masses, the change in the electrical output of said transducer will be substantially proportional to the change in the relative displacement between said masses.
 2. The improvement as set forth in claim 1 wherein said first mass is the body of the vehicle.
 3. The improvement as set forth in claim 1 wherein said second mass is a road wheel of the vehicle.
 4. The improvement as set forth in claim 1 wherein said transducer is a linear variable differential transformer, and wherein said one part is a core and said other part is a coil.
 5. The improvement as set forth in claim 1 wherein said first mass is sprung and said second mass is unsprung.
 6. The improvement as set forth in claim 1 wherein said spring is a coil spring. 